In the prior art, in section 6.2 of the G.8080 standard of the ITU-T for ITU Telecommunication Standardization Sector (ITU-T), an automatically switched optical network (ASON) transport plane is divided into routing areas and subnets, where one routing area may be composed of a number of sub-areas or subnets. The function of hierarchical routing may be implemented on the ASON transport plane by dividing routing areas. The G.17715 standard describes systematically the implementation of the hierarchical routing, the main idea of which is to convert the route between a pair of nodes belonging to different areas respectively into hierarchical routes. As shown in FIG. 1, the route from a node A in an area 1 to a node B in an area 4 is divided into an intra-area route from the node A to the boundary of the area 1, an inter-area route from the area 1 to the area 4 and an intra-area route from the boundary of the area 4 to the node B. In this figure, the intra-area route is indicated by a thin line and the inter-area route is indicated by a thick line. Thus, route-finding of a cross-area connection can be implemented by the inter-area route provided by a node on the boundary of the area 1 and the intra-area route to the node B provided by a node on the boundary of the area 4 on the other side, provided that the node A has the route to the boundary of the area 1.
Therefore, the advantage of dividing areas is that details of network topology outside the areas can be shielded, each area is only required to maintain its own internal network topology, and by link aggregation, connections between the areas are maintained and inter-area routes are generated, thereby decreasing the consumption of the CPU of a ASON node in calculating topology routes and increasing the convergence speed of the network.
The concept of area hierarchical routing of the ASON transport plane is mentioned in both the G.67715 standard and the G.8080 standard, but neither of the standards involves a reference to a partitioned area. In fact, the area hierarchical routing of the ASON transport plane is similar to the area routing aggregation theory of the Open Shortest Path First (OSPF) standard routing protocol. The OSPF standard protocol RFC2328 discusses specially the concept of a partitioned area in section 3.7 “Partitions of areas”, and provides measures to solve the problem of partitioned area connectivity. It is discovered by comparison that for the area hierarchical routing theory of the ASON transport plane, it is necessary to consider an implementation method of the hierarchical routing in the case of a partitioned area.